Limits to maize productivity in Western Corn-Belt: A simulation analysis for fully irrigated and rainfed conditions

Unlike the Central and Eastern U.S. Corn-Belt where maize is grown almost entirely under rainfed conditions, maize in the Western Corn-Belt is produced under both irrigated (3.2millionha) and rainfed (4.1millionha) conditions. Simulation modeling, regression, and boundary-function analysis were used to assess constraints to maize productivity in the Western Corn-Belt. Aboveground biomass, grain yield, and water balance were simulated for fully irrigated and rainfed crops, using 20-year weather records from 18 locations in combination with actual soil, planting date, plant population, and hybrid-maturity data. Mean values of meteorological variables were estimated for three growth periods (pre- and post-silking, and the entire growing season) and used to identify major geospatial gradients. Linear and stepwise multiple regressions were performed to evaluate variation of potential productivity in relation to meteorological factors. Boundary functions for water productivity and water-use efficiency were derived and compared against observed data reported in the literature. Geospatial gradients of seasonal radiation, temperature, rainfall, and evaporative demand along the Western Corn-Belt were identified. Yield potential with irrigation did not exhibit any geospatial pattern, depending instead on the specific radiation/temperature regime at each location and its interaction with crop phenology. A linear and parabolic response to post-silking cumulative solar radiation and mean temperature, respectively, explained variations on yield potential. Water-limited productivity followed the longitudinal gradient in seasonal rainfall and evaporative demand. Rainfed crops grown in the Western Corn-Belt are frequently subjected to episodes of transient and unavoidable water stress, especially around and after silking. Soil water at sowing ameliorates, but does not eliminate water stress episodes. Boundary functions for water productivity had slopes of 46 and 28kgha⁻¹ mm⁻¹, for aboveground biomass and grain yield, respectively. At high seasonal water supply, productivity was weakly correlated with water supply because many crops did not fully utilize seasonally available water due to percolation below the root zone or water left in the ground at physiological maturity. Fitted boundary functions for water-use efficiency had slopes ([almost equal to]seasonal transpiration-efficiency) of 54 and 37kgha⁻¹ mm⁻¹ for aboveground biomass and grain yield, respectively, and an x-intercept around 25-75mm ([almost equal to]seasonal soil evaporation). Data collected from experiments conducted in low-rainfall environments indicated that the boundary functions for water-use efficiency, derived from this study, are broadly applicable.